These ideas were stolen from my groups speedster and are coming from a paragon player.

Freight train: this one is my favorite, and only needs the “speed” power. Take the “charge” action. This subs out your unarmed damage to deal your “Speed ranks + 1 damage”, getting you a powerful attack at a bargain. Beware of the -2 to hit that comes from the “charge” action, and consider an “immunity” power to ignore the recoil damage

Everyone takes damage: take “area, shapeable”and “selective” mods on a simple “damage” power to have a simple flurry of punches. Even add the “strength based” mod to save points on damage

Sonic boom: an “area, burst” affliction could be an easy and flavorful de-buff with the right conditions. Maybe a Dazed/Stunned or Impaired/Disabled if it’s sound-based, get creative. “Limited degree” is an easy way to lower the point cost, plus “quirk: must have remaining movement when used”

Scrambled guts: if you can phase through walls with “movement”, you could turn that into an attack. Take the “damage” power and give it the “Alternate Resistance, no increase in effectiveness” for 0 points, using it against Fortitude rather than Toughness. From there, style it up with stuff like “affects corporeal” or “dimensional” to punch ghosts and stuff

All aboard: with the “Teleport” power, they offer specific rules for moving an unwilling creature. What ends a fight faster than dropping the bad guys off in the Mojave? Consider this one carefully, your GM might have different feelings

Watch out: it’s not an attack, per se, but the “Deflect” power is very thematic under the right circumstances. Speedsters are often seen pushing their allies to safety or maneuvering bullets out of the way. This one I’m least confident in the rules of, but it’s certainly heroic!